Catalysts For Low-temperature CO Oxidation Research Articles

Unravelling the Cu and Ce Effects in MnO2-Based Catalysts for Low-Temperature CO Oxidation

Cu-containing and Ce-modified OMS-2 catalysts were prepared at various calcination temperatures using the hydrothermal method and tested for low-temperature CO oxidation. The structure, chemical compositions, and physical–chemical properties of the catalysts were characterized using XRD, N2 physisorption, XRF, Raman spectroscopy, SEM, high-resolution TEM with EDX, TPR-H2, and XPS. The incorporation of Cu into the Ce-OMS-2 sample facilitated the transformation of pyrolusite into cryptomelane, as confirmed by Raman spectroscopy data. In the light-off mode, the Cu/Ce-OMS-2-300 and Cu/OMS-2 samples exhibited higher activity in low-temperature CO oxidation (T90 = 115 and 121 °C, respectively) compared to sample Cu/Ce-OMS-2-450. After a long-run stability test, the Cu/Ce-OMS-X samples demonstrated excellent performance: the T80 increased by 16% and 7% for the samples calcined at 300 °C and 450 °C, respectively, while the T80 for the Cu/OMS-2 increased by 40%. The Cu/OMS-2 and Cu/Ce-OMS-2-300 samples were found to have an increased content of nanodispersed copper sites on their surfaces. These copper sites contributed to the formation of the Cu2+-O-Mn4+ interface, which is responsible for the CO oxidation. The presence of Ce3+ in the catalyst was found to increase its stability in the presence of water vapor due to the higher reoxidation ability in comparison with Ce-free sample Cu/OMS-2.

Enhancing Activity and Stability of Pd-on-TiO2 Single-Atom Catalyst for Low-Temperature CO Oxidation through in Situ Local Environment Tailoring.

The development of efficient Pd single-atom catalysts for CO oxidation, crucial for environmental protection and fundamental studies, has been hindered by their limited reactivity and thermal stability. Here, we report a thermally stable TiO2-supported Pd single-atom catalyst that exhibits enhanced intrinsic CO oxidation activity by tunning the local coordination of Pd atoms via H2 treatment. Our comprehensive characterization reveals that H2-treated Pd single atoms have reduced nearest Pd-O coordination and form short-distanced Pd-Ti coordination, effectively stabilizing Pd as isolated atoms even at high temperatures. During CO oxidation, partial replacement of the Pd-Ti coordination by O or CO occurs. This unique Pd local environment facilitates CO adsorption and promotes the activity of the surrounding oxygen species, leading to superior catalytic performance. Remarkably, the turnover frequency of the H2-treated Pd single-atom catalyst at 120 °C surpasses that of the O2-treated Pd single-atom catalyst and the most effective Pd/Pt single-atom catalysts by an order of magnitude. These findings open up new possibilities for the design of high-performance single-atom catalysts for crucial industrial and environmental applications.

Unveiling the mechanistic synergy in Mn-doped NiO catalysts with atomic-burry structure: Enhanced CO oxidation via Ni-OH and Mn bifunctionality

The development of non-noble metal catalysts for low-temperature CO oxidation is a pivotal subject in various disciplines. In this study, an optimized Mn-doped NiO catalyst (Mn-NiO-0.5), prepared via aerosol pyrolysis with an equal molar ratio of Ni to Mn, demonstrates remarkable performance. It achieves an 82 % CO conversion at -20 °C and a 100 % conversion between 10 °C and 800 °C. The catalyst’s turnover frequency (TOF) for CO oxidation significantly surpasses that of previously reported noble-metal-based catalysts, underscoring its superior efficiency. Dynamic analyses suggest that the enhanced catalytic activity is a result of the unique atom burr structure, which presents an extensive surface area replete with abundant active sites. This structural feature is instrumental in maximizing the catalyst’s interaction with CO molecules. Experimental findings, corroborated by density functional theory (DFT) calculations, elucidate the mechanism of CO oxidation on the Mn-NiO-0.5 catalyst. The process is facilitated by the synergistic action of bimetallic sites, where the Ni–OH site acts as the primary adsorption point for CO. Subsequently, the CO is oxidized to bicarbonate through the interaction with the oxygen (O or O2) at adjacent Mn sites. This pioneering study introduces a paradigm-shifting insight: the effectiveness of non-noble metal oxide catalysts can now compete with, and in some cases, outperform their noble metal counterparts. This breakthrough is set to transform the industry, presenting a compelling alternative to conventional noble metal catalysts and propelling the evolution of state-of-the-art environmental conservation technologies.

Unveiling the Promoting Mechanism of H2 Activation on CuFeOx Catalyst for Low-Temperature CO Oxidation.

The effect of H2 activation on the performance of CuFeOx catalyst for low-temperature CO oxidation was investigated. The characterizations of XRD, XPS, H2-TPR, O2-TPD, and in situ DRIFTS were employed to establish the relationship between physicochemical property and catalytic activity. The results showed that the CuFeOx catalyst activated with H2 at 100 °C displayed higher performance, which achieved 99.6% CO conversion at 175 °C. In addition, the H2 activation promoted the generation of Fe2+ species, and more oxygen vacancy could be formation with higher concentration of Oα species, which improved the migration rate of oxygen species in the reaction process. Furthermore, the reducibility of the catalyst was enhanced significantly, which increased the low-temperature activity. Moreover, the in situ DRIFTS experiments revealed that the reaction pathway of CO oxidation followed MvK mechanism at low temperature (<175 °C), and both MvK and L-H mechanism was involved at high temperature. The Cu+-CO and carbonate species were the main reactive intermediates, and the H2 activation increased the concentration of Cu+ species and accelerated the decomposition carbonate species, thus improving the catalytic performance effectively.

Revealing of K and SO2 poisoning mechanism on CuCeOx catalyst for low-temperature CO oxidation

The application of CuCeOx catalysts for CO elimination in industrial sintering flue gases containing alkali metals is still a challenge. The effect mechanism of K species or/and SO2 poisoning on low temperature CO oxidation of CuCeOx catalyst was investigated. The results illustrated that the activity of CC-K2O catalyst was lower than that of CC-KCl catalyst, and the co-poisoning of K species and SO2 had more serious influence on CO conversion. The K species or/and SO2 poisoning resulted in the decrease of specific surface area and pore volume. In addition, K species or/and SO2 poisoning reduced the concentrations of Cu+, Ce3+ and chemisorbed oxygen species. Besides, K species or/and SO2 poisoning decreased the amount of oxygen vacancies and impaired the reducibility, which inhibited adsorption activation of oxygen and redox cycle. Furthermore, the CO oxidation on CC catalyst followed both MvK mechanism and L-H mechanism. The hydroxyl groups were involved in the reaction as an active oxygen species to generate active intermediate of bicarbonate which was easily decomposed. The KCl and K2O poisoning displayed fewer carbonyl and carbonate active intermediates, and the participation of hydroxyl groups in CO oxidation was inhibited. The co-existence of K species and SO2 further reduce the production of active intermediates, and the regeneration of hydroxyl was also blocked, thus resulting in lower activity.

Cooperation of Mn and P atoms in graphene for efficiently catalyzing CO oxidation at low temperature

Computational studies show that the co-doping of Mn and P atoms in graphene (MnP4-Gr) can provide high CO oxidation performance at room temperature. Results suggest that the configuration (CO+O2)* can be easily obtained due to the better adsorption of CO on the Mn atom and O2 on the P atom. (CO+O2)* is further found to undergo a fast Langmuir−Hinshelwood (LH) process and then left one atomic oxygen on P which is difficult to remove. Therefore, due to the more favorable adsorption of CO on the Mn atom and O2 on other P atoms, coupled with the speed of the LH process, the configuration (CO+4O)* of one atomic oxygen adsorbed on each P atom and CO on Mn atom is finally obtained. The whole reaction is determined by the atomic oxygen removal in (CO+4O)* with an energy barrier of 0.80 eV. It is predicted that the cooperation of Mn and P atoms is conducive to the fast LH process of MnP4-Gr and also the weaker binding between P and O in MnP4-Gr. Moreover, thermodynamic and kinetics analysis indicates that increasing temperature can accelerate CO oxidation and facilitate reaction in thermodynamics. Therefore, the insights reported in this paper will provide a new catalyst for low-temperature CO oxidation.

Supported gold nanoparticles prepared from NHC-Au complex precursors as reusable heterogeneous catalysts

Gold nanoparticles supported on SiO2, Al2O3, TiO2, and carbon (Maxsorb, Ketjen black and Shirasagi New Gold) were prepared by impregnation and solid grinding methods using N-heterocyclic carbene (NHC)-Au complexes as precursors. A total of nine NHC-Au complex precursors were tested by impregnation and solid grinding method, and [Au(OH)(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)1,3-dihydro-2H-imidazol-2-ylidene) complex led to (1–2 nm) particle sizes by both methods. The prepared catalysts were characterized by XRD (X-ray diffraction), HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy), in situ XAFS (X-ray absorption fine structure), etc. In addition, [Au(OH)(IPr)] and its impregnated [Au(OH)(IPr)]/SiO2 were subjected to TG-DTA (thermogravimetry analysis-differential thermal analysis) and in situ XAFS analysis under calcination conditions, respectively. Moreover, the obtained supported Au nanoparticles (NPs) were effective as catalysts for low-temperature CO oxidation, intramolecular cyclization of alkynyl carboxylic acids, and isomerization of allylic esters. Their results suggested that the reduction of [Au(OH)(IPr)] proceeds rapidly from around 300°C, and the residual ligands should be attached to the surface of the Au NPs and protect them from drastic agglutination. These results reveal that properly designed NHC-Au complexes can facilitate the preparation of supported Au NP catalysts with small particle sizes and high activities.

In situ DRIFTS and TPD studies on surface properties affecting SO2-resistance of Pt/TiO2 catalyst in low-temperature CO oxidation

By pre-calcining two TiO2 (P25 and anatase) supports at various temperatures, Pt/TiO2 of different Pt valence states, surface hydroxyls (OH), and peroxide contents were prepared. Pt/Anatase 450 and Pt/P25 450, whose supports were pre-calcined at 450 °C, performed excellently in the CO oxidation catalytic activity test using flue gas without water and SO2. Their low T50 (165 and 175 °C, respectively) and high TOF at 125 °C (0.204 and 0.247 s−1, respectively) of CO oxidation are attributed to the low stability of the adsorbed CO on Pt sites. Furtherly, in the catalytic test using water and SO2-containing flue gas, the catalysts using anatase supports achieved higher performance than P25 supports, showing T50 of 182−186 °C and TOF of 0.202−0.207 s−1 at 165 °C. The in situ DRIFTS study and TPD analysis indicate that OH participates in the sulfate formation. The limited hydroxylation on the anatase surface provides a small amount of OH, thereby mitigating the stable sulfate accumulation. Less stable sulfate on anatase than P25 gives rise to the high activity of anatase-supported catalysts.

Effect of water content and storage conditions on the activity of PdCuFe-containing fibrous carbon catalyst for low-temperature CO oxidation

Effect of water content and storage conditions on the activity of carbon fiber supported palladium-copper-iron containing catalyst was studied. Physical-chemical properties of catalytic system were examined by means of XRD, XPS, SEM and GCh. The optimal water content in the catalyst, which ensures the most effective air purification both at low (0,03 vol.%) and high (0,5 vol.%) CO concentrations in the air and high humidity (70 %) of gas mixture, was determined.Activity loss of the catalytic system after storage under air conditions (1 month) was attributed to the accumulation of the excess amount of water in the catalyst and reorganization of its the active phase – decrease in the palladium content in the near-surface layer and segregation of the Сu2Cl(OH)3 atacamite phase. Catalyst drying in air at 110 оС leads to its complete reactivation. Hermetically sealed freshly as-prepared and dried at 50 оС after experiment samples retain the initial catalytic activity during long-term (more than a year) storage.

Probing the redox capacity of Pt–CeO2 model catalyst for low-temperature CO oxidation

The redox activity of the Pt–CeO2 catalysts is controlled by CO spillover in the low-temperature region and by Mars–van Krevelen mechanism in the high-temperature region.

Atomically Incorporating Ni into Mesoporous CeO2 Matrix via Synchronous Spray-Pyrolysis as Efficient Noble-Metal-Free Catalyst for Low-Temperature CO Oxidation

Low-temperature catalytic CO oxidation is an important chemical process in versatile applications, such as the H2 utilization for low-temperature H2 air fuel cells. Pt-group metal catalysts are efficient but highly cost-consuming. This work demonstrates an excellent and sixpenny catalyst with earth-abundant Ni and Ce, in which Ni ions are atomically incorporated into the CeO2 matrix (Ni-Ce-Ox) by synchronous spray-pyrolysis (SSP) of mixture nitrates of Ni and Ce. The Ni-Ce-Ox catalyst presents a mesoporous structure. Revealed by a model reaction of 1% CO, 1% O2, and 98% balance He at a space velocity of 13,200 mL/gcat/h, Ni-Ce-Ox catalysts display a typical volcano-shaped relationship between reactivity and Ni incorporation amount. The optimized Ni incorporation appears with a high Ni/Ce atomic ratio of 0.25, endowing the T50 (temperature corresponding to a CO conversion of 50%), which is lower-shifted by 165 °C than that of pristine CeO2 (266 °C). The density functional theory (DFT) calculations further indicate that the much-reduced oxygen vacancy formation energy at Ni-Ce single-atom sites boosted the adsorption activation of the CO molecule and therefore promoted the CO oxidation process. Besides, the2 Ni-Ce-Ox from the SSP method presents better performance than the counterparts from immersion and hydrothermal methods. This work paves a way to access efficient noble-metal-free catalysts for low-temperature CO oxidation.

Low-temperature CO oxidation on CuO-CeO2 catalyst prepared by facile one-step solvothermal synthesis: Improved activity and moisture resistance via optimizing the activation temperature

Development of a highly efficient Cu-based catalyst for low-temperature CO oxidation under moisture condition remains a grand challenge. In this paper, effect of activation temperature (350–800 °C) on catalytic performance of CuO-CeO2, prepared by facile one-step solvothermal synthesis, in CO oxidation was investigated, and the catalysts’ physicochemical properties were studied by TG-DSC, ICP-AES, N2 sorption, XRD, TEM, Raman spectroscopy, XPS, in-situ DRIFTS, H2-TPR, CO-TPD, and O2-TPD. It was found that activation at 550 °C is beneficial for the interaction between CuO and CeO2, leading to the formation of more Cu+ species and oxygen vacancies advantageous for improving the catalytic activity. Activation at 800 °C, nevertheless, would weaken the interaction between CuO and CeO2 due to the sintering of CuO and CeO2, thus decreasing the catalytic activity and moisture resistance. Interestingly, no deactivation was observed over the optimal catalyst (activated at 550 °C) after 50 h testing at a low temperature of 87 °C (at which 90% CO conversion was obtained), even when 4.2% water vapor was added into the feed gas.

Fluidized-bed plasma enhanced atomic layer deposition of Pd catalyst for low-temperature CO oxidation

A fluidized-bed plasma-enhanced atomic layer deposition (FP-ALD) process is reported to fabricate Pd nanoparticles using palladium hexafluoroacetylacetonate and H2 plasma. The process successfully deposits Pd nanoparticles over porous γ-Al2O3 (30 wt. %), amorphous aluminum silicate (50 wt. %), and molecular sieve (20 wt. %) (ASM) powders. Pd loading on ASM is increased linearly with increasing the number of FP-ALD cycle with a growth rate of 0.34 mg/1 g ASM/cycle. Transmission electron microscopy reveals that high-density Pd nanoparticles are uniformly distributed over the entire ASM powders and the average Pd particle size is sensitive to the number of FP-ALD cycle. By increasing the number of FP-ALD cycles from 25 to 150, the average Pd particle size rises from 0.9 to 5.8 nm, indicating the particle size can be tuned easily by varying the number of FP-ALD cycles. The catalytic activities of different particle sizes and Pd loading samples are evaluated for CO oxidation. With the metal loading amount of 2% for Pd and the average particle size of 2.9 nm, the deposited Pd/ASM sample shows an excellent catalytic activity for the oxidation of CO. Under the condition of a gas mixture of 0.5 vol. % CO and 21 vol. % O2 balanced with N2, and gas hourly space velocity of 24 000 h−1, 100% CO conversion temperature is as low as 140 °C.

Mechanistic study of low-temperature CO oxidation over CuO/Cu2O interfaces with oxygen vacancy modification

Copper oxides have attracted great attention in the oxidation of CO due to their high activity at low temperatures. In this work, novel CuO/Cu2O heterojunction catalysts for low-temperature CO oxidation was prepared via hydrothermal in-situ oxidizing in H2O2. Based on studies of surface composition and the corresponding catalytic activities, the CuO/Cu2O catalysts contain rich oxygen vacancies (OVs), which improve the activation and migration ability of O2. The coupling effect between OVs and heterojunction reduces the activation energy of CO oxidation, leading the temperature of CO completed oxidation temperature from 250 °C to 140 °C. In addition, the in-situ Fourier transform infrared spectroscopy (in-situ FTIR) and Density functional theory (DFT) studies confirm two reaction paths under Langmuir-Hinshelwood (LH) mechanism: a low barrier path takes place at low temperature, and another new reaction path with a barrier of 1.8 eV can take place at a high temperature. The new reaction site formed at the CuO/Cu2O interface resulted in different activation modes of CO and O2. The modification of OVs significantly reduces the reaction temperature required. This work provides a method to improve the performance of co-catalytic oxidation via the coupling effect of OVs and heterojunctions.

Catalysts for Low-Temperature CO Oxidation Based on Platinum, CeO2, and Carbon Nanotubes

Catalysts for Low-Temperature CO Oxidation Based on Platinum, CeO2, and Carbon Nanotubes

Active sites and deactivation of room temperature CO oxidation on Co3O4 catalysts: combined experimental and computational investigations

Co3O4 is a well-known low temperature CO oxidation catalyst, but it often suffers from deactivation. We have thus examined room temperature (RT) CO oxidation on Co3O4 catalysts by operando DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO2. Catalysts pretreated in oxygen at 400 °C are most active, with the initial interaction of CO and Co3O4 being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high RT activity then levels-off, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co3O4 surface that upon reaction onset loses its most active oxygen. This specific active oxygen is not reestablished by gas phase O2 during the RT reaction. When the reaction temperature is increased to 150 °C, full conversion can be maintained for 100 h, and even after cooling back to RT. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational modeling using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at RT was identified, which may block a vacancy site from further reaction unless CO is removed at higher temperature. The interaction between oxygen vacancies was found to be small, so that in the active state several lattice oxygen species are available for reaction in parallel.

Co supported on Cex[Al2O3]0.5-x as an effective catalyst for low-temperature CO oxidation: Effect of calcination temperature

The low-temperature behavior of CO oxidation was investigated using 0.1 mol Ce doped Co supported on alumina catalysts prepared using co-precipitation followed by deposition precipitation method. The impact of calcination temperature on CO oxidation with kinetic aspects has been studied in detail and correlated to the characterizations. The Ce doped CoAl catalyst calcined at optimum temperature (550 ⁰C) shoed formation of more facile Co3+, Ce3+, and adsorbed species. The Co, Ce, and alumina interaction leads to the formation of optimum penta and hexa coordinated alumina. The physicochemical properties of the catalyst have been studied in detail using XRD, FTIR, Raman, TPR, XPS, and 27Al NMR and correlated to the catalytic activity.

Surface pits stabilized Au catalyst for low-temperature CO oxidation

Surface pits stabilized Au catalyst for low-temperature CO oxidation

Stability of Pt-Adsorbed CO on Catalysts for Room Temperature-Oxidation of CO

A large signal of gas-phase CO overlapping with those of adsorbates is often present when investigating catalysts by operando diffuse reflectance FT-IR spectroscopy. Physically removing CO(g) from the IR cell may lead to a fast decay of adsorbate signals. Our work shows that carbonyls adsorbed on metallic Pt sites fully vanished in less than 10 min at 30 °C upon removing CO(g) when redox supports were used. In contrast, a broad band assigned to CO adsorbed on oxidized Pt sites was stable. It was concluded that physically removing CO(g) at room temperature during IR analyses will most likely lead to changes in the distribution of CO(ads) and a misrepresentation of the Pt site speciation, misguiding the development of efficient low-temperature CO oxidation catalysts. A tentative representation of the nature of the Pt phases present depending on the feed composition is also proposed.

Highly Active CuO/KCC−1 Catalysts for Low-Temperature CO Oxidation

Copper catalysts have been extensively studied for CO oxidation at low temperatures. Previous findings on the stability of such catalysts, on the other hand, revealed that they deactivated badly under extreme circumstances. Therefore, in this work, a series of KCC−1-supported copper oxide catalysts were successfully prepared by impregnation method, of which 5% CuO/KCC−1 exhibited the best activity: CO could be completely converted at 120 °C. The 5% CuO/KCC−1 catalyst exhibited better thermal stability, which is mainly attributed to the large specific surface area of KCC−1 that facilitates the high dispersion of CuO species, and because the dendritic layered walls can lengthen the movement distances from particle-to-particle, thus helping to slow down the tendency of active components to sinter. In addition, the 5% CuO/KCC−1 has abundant mesoporous and surface active oxygen species, which are beneficial to the mass transfer and promote the adsorption of CO and the decomposition of Cu+–CO species, thus improving the CO oxidation performance of the catalyst.